NE Netherlands Research Articles

Temperature and Gas/Brine Content Affect Seismogenic Potential of Simulated Fault Gouges Derived From Groningen Gas Field Caprock

AbstractWe investigated the rate‐and‐state frictional properties of simulated anhydrite‐carbonate fault gouge derived from the basal Zechstein caprock overlying the seismogenic Groningen gas reservoir in the NE Netherlands. Direct shear experiments were performed at in situ conditions of 50–150 °C and 40‐MPa effective normal stress, using sliding velocities of 0.1–10 μm/s. Reservoir pore fluid compositions were simulated using 4.4 Molar NaCl brine, as well as methane, air, and brine/gas mixtures. Brine‐saturated samples showed friction coefficients (μ) of 0.60–0.69, with little dependence on temperature, along with velocity strengthening at 50–100 °C, transitioning to velocity weakening at 120 °C and above. By contrast, gas filled, evacuated and partially brine‐saturated samples showed μ values of 0.72 ± 0.02 plus strongly velocity‐weakening behavior accompanied by stick slip at 100 °C (the only temperature investigated for gas‐bearing and dry samples). A microphysical model for gouge friction, assuming competition between dilatant granular flow and thermally activated compaction creep, captures the main trends seen in our brine‐saturated samples but offers only a qualitative explanation for our gas‐bearing and dry samples. Since the reservoir temperature is ~100 °C, our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal Zechstein anhydrite‐carbonate caprock against the Groningen reservoir sandstone, specifically in the gas‐filled upper portion of the reservoir system.

Structural and microstructural analysis of K–Mg salt layers in the Zechstein 3 of the Veendam Pillow, NE Netherlands: development of a tectonic mélange during salt flow

AbstractIn fully developed evaporite cycles, effective viscosity contrasts of up to five orders of magnitude are possible between different layers, but the structures and mechanics in evaporites with such extreme mechanical stratification are not well understood. The Zechstein 3 unit in the Veendam salt pillow in the Netherlands contains anhydrite, halite, carnallite and bischofite, showing this extreme mechanical stratification. The Veendam Pillow has a complex multiphase salt tectonic history as shown by seismic reflection data: salt withdrawal followed by convergent flow into the salt pillow produced ruptures and folds in the underlying Z3-anhydrite–carbonate stringer and deformed the soft Z3-1b layerWe analysed a unique carnallite- and bischofite-rich drill core from the soft Z3-1b layer by macroscale photography, bulk chemical methods, X-ray diffraction and optical microscopy. Results show high strain in the weaker bischofite- and carnallite-rich layers, with associated dynamic recrystallisation at very low differential stress, completely overprinting the original texture. Stronger layers formed by alternating beds of halite and carnallite show complex recumbent folding on different scales commonly interrupted by sub-horizontal shear zones with brittle deformation, veins and boudinage. We attribute this tectonic fragmentation to be associated with a softening of the complete Z3-1b subunit during its deformation. The result is a tectonic mélange with cm- to 10 m-size blocks with frequent folds and boudinage. We infer that these structures and processes are common in deformed, rheologically strongly stratified evaporites.

Frictional Properties of Simulated Fault Gouges from the Seismogenic Groningen Gas Field Under In Situ P–T ‐Chemical Conditions

AbstractWe investigated the frictional properties of simulated fault gouges derived from the main lithologies present in the seismogenic Groningen gas field (NE Netherlands), employing in situ P–T conditions and varying pore fluid salinity. Direct shear experiments were performed on gouges prepared from the Carboniferous shale/siltstone substrate, the Upper Rotliegend Slochteren sandstone reservoir, the overlying Ten Boer claystone, and the Basal Zechstein anhydrite–carbonate caprock, at 100°C, 40 MPa effective normal stress, and sliding velocities of 0.1–10 μm/s. As pore fluids, we used pure water, 0.5–6.2 M NaCl solutions, and a 6.9 M mixed chloride brine mimicking the formation fluid. Our results show a marked mechanical stratigraphy, with a maximum friction coefficient (μ) of ~0.66 for the Basal Zechstein, a minimum of ~0.37 for the Ten Boer claystone, ~0.6 for the reservoir sandstone, and ~0.5 for the Carboniferous. Mixed gouges showed intermediate μ values. Pore fluid salinity had no effect on frictional strength. Most gouges showed velocity‐strengthening behavior, with little systematic effect of pore fluid salinity or sliding velocity on (a–b). However, Basal Zechstein gouge showed velocity weakening at low salinities and/or sliding velocities, as did 50:50 mixtures with sandstone gouge, tested with the 6.9 M reservoir brine. From a rate and state friction viewpoint, our results imply that faults incorporating Basal Zechstein anhydrite–carbonate material at the top of the reservoir are the most prone to accelerating slip, that is, have the highest seismogenic potential. The results are equally relevant to other Rotliegend fields in the Netherlands and N. Sea region and to similar sequences globally.

Impact of technical and economic uncertainties on the economic performance of a deep geothermal heat system

This paper presents a techno-economic analysis of a deep, direct use geothermal heat system in a conductive geological setting (Groningen, NE Netherlands). The model integrates the previously discussed uncertainties of the initial reservoir state, geological and operational conditions with the economic uncertainties. These uncertainties are incorporated in the form of probability distributions and 20,000 iterations of the model are performed over a project lifetime of 40 years. A combination of Ex-Ante and Ex-Post criteria are used to evaluate the economic performance of the system based on the Net Present Value (NPV), Levelised Cost of Heat (LCOH) and Expected Monetary Value (EMV). The sensitivity analysis highlights the load factor (effective flowrate) as the most important parameter for the economic performance and energy costs. However, the differences between the NPV and LCOH sensitivities highlight the importance of using both metrics for the economic performance of such systems. The presented project remains economically challenging, exhibiting a 50% probability of marginal revenues over its lifetime. Systematic insights are drawn with regard to potential improvements of technical and economic aspects of such geothermal heat systems.

Salt intrusions providing a new geothermal exploration target for higher energy recovery at shallower depths

Direct use of geothermal energy can present challenges of financial feasibility in a low-enthalpy setting. The average temperature gradients in sedimentary basins make it necessary to reach larger depths for meaningful heat production, thus increasing the drilling cost. Therefore, full realization of geothermal projects in low-enthalpy environments has been difficult and not widely deployed. The concept of harvesting the positive temperature anomalies caused by the increased heat conductivity of salt bodies could enable access to higher temperatures at a shallower depth, thus reducing the necessary depth of drilling. In a potential site in NE Netherlands, temperature differences of up to 25 °C close to the top of a salt body are modeled. Substantiating this concept we show that the energetic benefits can result to up to 40% more energy extracted, while the temperature recovery of the field is only prolonged by 13%. This opens up new possibilities for geothermal applications in sedimentary basins.

Risk assessment of the Groningen geothermal potential: From seismic to reservoir uncertainty using a discrete parameter analysis

Geothermal exploitation is subject to several uncertainties, even in settings with high data availability, adding to project risk. Uncertainty can stem from the reservoir’s initial state, as well as from the geological and operational parameters. The interplay between these aspects entails irreducible risk prior to exploration drilling. Consequently it is difficult to construct an indicative qualitative and quantitative depiction of the most prominent facets (e.g. pressure, permeability). This paper shows the classification of known unknowns to risks, while also providing numerical results. Starting from seismic data and arriving at a reservoir model using a discrete parameter analysis we assess the risks and uncertainties of a geothermal project near the city of Groningen (NE Netherlands). By simulating all combinations of the considered parameters, their relative importance can be mapped out. Findings suggest that the unique regime of possible pressure depletion due to neighbouring gas production can highly impact the feasibility of the project. Results demonstrate how an in depth analysis at the exploration phase can direct future efforts towards the most significant elements. Although the numerical results are field specific, the methodology can be readily applied to different locations.

Evolution of rheologically heterogeneous salt structures: a case study from the NE Netherlands

Abstract. The growth of salt structures is controlled by the low flow strength of evaporites and by the tectonic boundary conditions. The potassium-magnesium salts (K-Mg salts) carnallite and bischofite are prime examples of layers with much lower effective viscosity than halite: their low viscosity presents serious drilling hazards but also allows squeeze solution mining. In contrast, intrasalt anhydrite and carbonate layers (stringers) are much stronger than halite. These rheological contrasts within an evaporite body have an important control on the evolution of the internal structure of salt, but how this mechanical layering affects salt deformation at different scales is not well known. In this study, we use high-resolution 3-D seismic and well data to study the evolution of the Veendam and Slochteren salt pillows at the southern boundary of the Groningen High, northern Netherlands. Here the rock salt layers contain both the mechanically stronger Zechstein III Anhydrite-Carbonate stringer and the weaker K-Mg salts, thus we are able to assess the role of extreme rheological heterogeneities on salt structure growth. The internal structure of the two salt pillows shows areas in which the K-Mg salt-rich ZIII 1b layer is much thicker than elsewhere, in combination with a complexly ruptured and folded ZIII Anhydrite-Carbonate stringer. Thickness maps of supra-salt sediments and well data are used to infer the initial depositional architecture of the K-Mg salts and their deformation history. Results suggest that faulting and the generation of depressions on the top Zechstein surface above a Rotliegend graben caused the local accumulation of bittern brines and precipitation of thick K-Mg salts. During the first phase of salt flow and withdrawal from the Veendam area, under the influence of differential loading by Buntsandstein sediments, the ZIII stringer was boudinaged while the lens of Mg salts remained relatively undeformed. This was followed by a convergence stage, when the K-Mg salt-rich layers were deformed within the inflating salt pillows. This deformation was strongly disharmonic and strongly influenced by folding of the underlying, ruptured ZIII stringer, leading to thickening and internal deformation of the K-Mg salt layers.

Case studies for integrated Care development in US Children’s Hospitals, acute, primary and community services in NE Netherlands, and whole of state services in Jersey, UK

Case studies for integrated Care development in US Children’s Hospitals, acute, primary and community services in NE Netherlands, and whole of state services in Jersey, UK

Importance of mineral surface areas in Rotliegend sandstones for modeling CO2–water–rock interactions

Investigations of Rotliegend sandstones from the NE Netherlands have been carried out to determine the mineralogical composition and spatial mineral distribution, specific mineral and whole rock surface areas and the integration of these results in geochemical water–rock–CO2 simulations. Therefore, two scenarios were considered: (1) volumetric rock composition (derived from point counting on thin sections) and (2) effective mineralogy (derived from 2D pore lining image analysis) data which represent the percentage amount of each mineral phase exposed within open (or accessible) pores. Specific mineral surface areas were defined for each mineral phase, respectively and integrated in both model scenarios. Based on point counting data the volumetric compositions of the sandstones are dominated by the mineralogy of detrital grains like quartz and K-feldspar. Authigenic minerals, esp. clays, volumetrically make up a small portion of the bulk rock volume only but are exposed due to their small crystal size (nm- to μm-scale) and a large specific surface area (13 to 195m2/g) in comparison to e.g. feldspar grains (0.03 to 1.79m2/g). Hence the size, distribution and accessibility of detrital and authigenic minerals regulate the availability of chemical species for water–rock interactions in the pore space. The effective mineralogy of the sandstones is a function of the presence of grain covering and pore-filling authigenic minerals. In the two Rotliegend sandstones studied, approximately 23% and 43% of the detrital grains are covered by cements blocking the grain surface areas accessible for potential water–rock interactions. These minerals are Fe-oxide, clay cutans (mainly illite), kaolinite, quartz, carbonate (dolomite, ankerite and siderite), sulfate (anhydrite and barite) and galena. The specific surface area (SSA) of each mineral was calculated using (electron) microscopy to determine the sample geometry (e.g. size, thickness) and standard density values. For the volumetric and effective mineralogy data the total specific surface area of the bulk rock was calculated by summing the specific mineral surface areas in relation to their relative content in the rock, as determined using X-ray diffraction. These results were compared to surface area values determined using low pressure N2 adsorption (BET method) of the bulk rock (mean value 1.4m2/g). It is shown that the SSA values using the effective mineralogy approach (mean value of 0.46m2/g) are closer to the whole rock BET than the SSA for volumetric mineralogy (0.05m2/g). These different values for the volumetric and the effective mineralogy show significant differences in geochemical modeling results: The initial amount of K-feldspar, kaolinite, hematite and carbonate controls the availability of species for clay mineral precipitation (e.g. Mg-smectite) during CO2 storage. Effective mineral carbonation is calculated using the volumetric compared to the effective mineralogy. Within the first 100years carbonate precipitation occurs for the volumetric mineralogy but is absent for the effective mineralogy. Opposing trends are observed for long-term reactions (>100years). Similarly, when using the effective mineralogy, the porosity increases over the entire simulation time of 10,000years while it initially decreases for the volumetric mineralogy followed by an increase and a net zero porosity change after 10,000years. This implies that different results for CO2 trapping can be expected, depending upon the mineral assemblage and its effective mineralogy.

Sedimentology and diagenesis as ‘hydrocarbon exploration tools’ in the Late Permian Zechstein-2 Carbonate Member (NE Netherlands)

Abstract Hydrocarbon exploration in The Netherlands has a chequered history from serendipitous oil shows via chance oil/ gas discoveries to finding the largest continental European oil field in 1943, followed by finding the largest gas field in the world in 1959. The present contribution traces the development of moderate to good porosity/permeability trends in depositional facies of Zechstein Stassfurt carbonates in a ‘gas play’ intermediate in significance between the above two plays but all in the northern part of The Netherlands. Various depositional facies in the Stassfurt carbonates were turned into ‘carbonate fabric units’ by diagenetic processes creating or occluding the porosity/permeability. This formed moderate to good gas reservoirs in barrier-shoal, open-marine shelf and proximal-slope carbonates in the subsurface of the province of Drenthe in the NE Netherlands. The diagenetic models forming these carbonate fabric units are linked to the variety of facies in a depositional model which shows explain and predicts the reservoir trends. Such depositional/diagenetic facies are ‘translated’ into characteristic petrophysical values recognisable on wire line logs in uncored wells, and in characteristic seismic expressions that show these trends in undrilled areas. This approach has been proven to be effective in delineating porosity trends, visualised by 3-D seismic in the Collendoornerveen field, and thus provides a new exploration ‘tool’ in hydrocarbon exploration .

6주간 야외용 운동기구를 사용한 저항운동이 노인 여성의 무릎신전근 기능 및 구조에 미치는 영향

The purpose of this study was to investigate the effect of leg extension exercises performed on outdoor resistance exercise machines on knee extension muscle strength and quadriceps muscle group cross sectional area (CSA) in elderly women. Two groups were recruited for this study, including an exercise group (EG: n=13, <TEX>$71.38{\pm}2.79$</TEX> yrs) and a control group (CG: n=5, <TEX>$73.4{\pm}5.94$</TEX>), In all subjects, maximum isometric and isokinetic muscle strength of knee flexion and extension were measured using an isokinetic dynamometer (Cybex(R) Humac Norm Testing & Rehabilitation System, USA). Quadriceps muscle group CSA were measured using MRI (Philps, Intera 1.5 T, NE Netherlands). The results of this study showed that post-intervention isometric knee extension peak torque value were higher than pre-intervention measures in the EG. However, the EG did not show improvement in quadriceps muscle group CSA, Also, no differences in the shift of optimal knee joint angle were observed between pre and post-intervention exercise. Outdoor leg extension exercise showed small increases in muscle strength in comparison to other resistance training exercises. The results of this study suggest that because outdoor leg extension exercise machines lack a progressive loading mechanism, significant increases in muscle strength may not be obtained.

RESERVOIR POTENTIAL OF A LACUSTRINE MIXED CARBONATE / SILICICLASTIC GAS RESERVOIR: THE LOWER TRIASSIC ROGENSTEIN IN THE NETHERLANDS

The Lower Triassic Rogenstein Member of the Buntsandstein Formation produces gas at the De Wijk and Wanneperveen fields, NE Netherlands and consists mainly of claystones with intercalated oolitic limestone beds. The excellent reservoir properties of the oolites (φ= 20‐30%; k = 5‐4000 mD) are predominantly controlled by depositional facies. Oolitic limestones are interpreted as the storm and wave deposits of a shallow, desert lake located in the Central European Buntsandstein Basin.The vertical sequence of lithofacies in the Rogenstein Member indicates cyclic changes of relative lake level. The reservoir rock is vertically arranged in a three‐fold hierarchy of cycles, recognised both in cores and wireline logs. These cycles are a key to understanding the distribution of reservoir facies, and are used as the basis for a high‐resolution sequence stratigraphic correlation of the reservoir units.Slight regional‐scale thickness variations of the Rogenstein Member (in the order of tens of metres) are interpreted as the effects of differential subsidence associated with the inherited Palaeozoic structural framework. The depositional basin can be subdivided into subtle palaeo‐highs and ‐lows which controlled facies distribution during Rogenstein deposition. Oolitic limestones show their greatest lateral extent and thickest development in the Middle Rogenstein during large‐scale maximum flooding.Potential reservoir rocks (decimetre to metres thick) are present in the NE Netherlands, in particular in the Lauwerszee Trough and the Lower Saxony Basin, where abundant gas shows of 200 ‐ 4000 ppm CH4 have been recorded. Preserved primary porosity is interpreted to be a result of rapid burial in subtle depositional palaeo‐lows in this area. The thickest, amalgamated oolite intervals (tens of metres thick) occur in the eastern part of the Central Netherlands Basin. Because of poor reservoir properties, other areas appear to be less promising in terms of Rogenstein exploration potential.

CHARACTERISATION OF EPEIRIC “LAYER‐CAKE” CARBONATE RESERVOIRS: UPPER MUSCHELKALK (MIDDLE TRIASSIC), THE NETHERLANDS

Upper Muschelkalk (Middle Triassic) carbonates produce natural gas at Coevorden field in the NE Netherlands. This is currently the only field which produces gas from this succession although several other prospects have been identified nearby. In order to help develop these hydrocarbons, this study proposes a facies and reservoir model of the Upper Muschelkalk in the NE Netherlands together with a regional framework intended to assist in further evaluation.Distribution of facies and reservoir properties of the Upper Muschelkalk carbonates in the NE Netherlands indicate deposition on a storm‐dominated epeiric ramp with a very low gradient. The predominantly muddy and marly lithofacies types in proximal and distal parts of the ramp gradually interfinger with a shoreline‐detached “shoal”‐like ooidal grainstone complex. The best reservoir quality (permeability up to 60 mD) is recognised within dolomitised peloid ooid grainstones. These are interpreted as high‐energy backshoal deposits. Reservoir quality decreases in the limestone‐dominated “shoal” facies and the muddier foreshoal facies. A four‐fold hierarchy of depositional cycles describes the systematic and thus predictable vertical variation in reservoir quality (permeability) and quantity (net‐to‐gross). High‐resolution correlation suggests that medium‐scale cycles (5 to 15 metres thick) can be traced for hundreds of kilometres. Small‐scale cycles (1 to 3 metres thick) are persistent for several tens of kilometres and have sheet‐like geometries. Individual reservoir units (several decimetres thick) appear to be laterally continuous over a maximum of a few kilometres although internal flow barriers might be expected. Mapping of Upper Muschelkalk thickness and facies has clearly defined backshoal, ”shoal and foreshoal facies belts with distinctly different reservoir characteristics. Typically, reservoir quality and quantity decrease with increasing thickness of the Upper Muschelkalk and the underlying Middle Muschelkalk halite. The systematic variations in thickness are apparently controlled by a combination of palaeogeography and palaeotectonics. The best reservoir quality and highest quantity is found on a palaeohigh characterised by a relatively thin Upper Muschelkalk succession and the absence of underlying halite. These features can also be recognised in seismic data.The results of this case study can also be applied in the integrated characterisation of similar epeiric carbonates constituting highly productive reservoirs in the Middle East, including the Khuff and Arab Formations.

Production from Muschelkalk carbonates (Triassic, NE Netherlands): unique play or overlooked opportunity?

The Triassic Muschelkalk carbonates are a unique play in NW Europe, producing hydrocarbons onshore The Netherlands. This paper aims at a comprehensive description of these relatively unknown reservoirs and speculates on additional play potential outside of the current production areas. The Muschelkalk Formation consists of partially porous Lower and Upper Muschelkalk carbonates separated by tight Middle Muschelkalk evaporites. The 100 m thick Lower Muschelkalk holds 1.9 BCM gas in the De Wijk Field. The reservoir is a limestone–dolomite–marl succession producing from several decimetres thick calcisilitic dolo-mudstones encased in tight lime-mudstones. The reservoir shows a stratified architecture composed of 16 cycles with thin but laterally continuous porous units. Good reservoir quality facies are concentrated in the upper third of these cycles. Porosities vary from 6 to 27%, (average 12%) and permeabilities range from 0.01 to 32 mD (average 0.06 mD). The reservoir rocks represent inner ramp sediments of a storm-dominated epeiric ramp. The 50 m thick Upper Muschelkalk contains 0.8 BCM gas in the Coevorden Muschelkalk Field. The reservoir consists of several decimetres thick peloidal-oolitic dolo-grain to packstones of good permeability interbedded with lower permeability dolo-mudstones to wackestones. Porosity varies from 5 to 29% (average 15%) and permeability ranges from 0.01 to 57 mD (average 1 mD). The permeable dolo-grainstones occur chiefly around peak transgression and early regression of the Upper Muschelkalk. The producing Upper Muschelkalk deposits represent the inner ramp section of a storm-dominated homoclinal ramp. Numerous gas shows in the Lower and Upper Muschelkalk, scattered over the onshore territory of The Netherlands, suggest possible missed pay and upside potential. The highest reservoir potential occurs in grainy dolomitic inner ramp deposits. These are likely to be located close to the existing fields in the NE Netherlands. Additionally, currently unrecognized potential could be present in southern Germany and eastern Poland.

Reservoir characteristics of intracontinental carbonate ramp deposits-Upper Muschelkalk, Middle Triassic, NE Netherlands

AbstractThe Upper Muschelkalk is an unusual reservoir in NW Europe, producing only in the Coevorden Muschelkalk field, onshore the Netherlands. Origin and nature of the gas producing intervals were poorly known. The objective of the paper is to provide a comprehensive description of facies, cyclicity and petrophysical characteristics. From this description a depositional and sequence stratigraphic model is proposed, which explains why there is gas production only from certain intervals of the sequence. Our investigation is based on seismic, core and open hole log data. It indicates that the reservoir consists of dolomites, which are either muddy lagoonal to sabkha, or grainy backshoal deposits. The best reservoir quality is encountered in peloidal-oolitic packstones to grainstones. These represent storm-dominated backshoal deposits and constitute the inner part of a homoclinal carbonate ramp. The succession shows a conspicuous hierarchical cyclicity. Porous backshoal deposits form during maximum transgression and early regression. However permeable, gas producing backshoal deposits only occur in the upper 15 to 20 m, which forms the large-scale regressive hemi-cycle of the Upper Muschelkalk. Better reservoir quality in the upper hemi-cycle is due to changes in grain type and early diagenesis. The investigation might serve as calibration point for further exploring the Upper Muschelkalk reservoir and its facies pattern in the NW European basin.

Facies, cyclicity and reservoir properties of the Lower Muschelkalk (Middle Triassic) in the NE Netherlands

Carbonate ramps are prolific hydrocarbon reservoirs, usually associated with grain-rich facies types. The present paper provides a sedimentological description and genetic interpretation of a gas reservoir located in mud-dominated carbonates. These deposits are part of the Lower Muschelkalk ramp system of Middle Triassic age.

Forward prediction of aeolian systems using fuzzy logic, constrained by data from recent and ancient analogues

AbstractAeolian sands are the main reservoir rock in some of the largest gas fields, such as the Shell-Exxon Groningen Field, operated by NAM. Although aeolian reservoirs have been studied for many years, there is still room for improvement in the predictive modeling of such reservoirs. A pilot project with this objective was initiated by SIEP B.V. in 1997, together with Heriot-Watt University in Edinburgh, UK and with Uppsala University, Sweden, to evaluate the factors influencing aeolian systems, and to formulate a forward model using ‘fuzzy logic’.The project was initiated to develop a fuzzy system for generic modeling of aeolian architectures. The key aims were to be able to predict the type, amount and distribution of major facies in generic aeolian systems and specifically to model regional-scale architecture in the sub-surface. Fuzzy rules and sets, which defined the behaviour of aeolian systems, were constructed and used to modify the pre-existing fuzzy modeling software which had been designed for shallow and deep marine systems. The modeling procedure used input data appropriate to the Rotliegend climate, and was validated by comparing the resulting models, in terms of thickness and spatial distribution of facies types, to well data from the Upper Rotliegend interval of the Lauwerszee Trough area, NE Netherlands (Figures 1 &amp; 2).

Experiments to predict the response of thematic mapper bands 3, 4 and 5 to grass drought.

Turf samples 60cm x30 cm and 10 cm deep from 3 locations in NE Netherlands showed a range of grass and soil types. During a drying period in the laboratory, reflectance measurements were carried out and moisture contents of grass and soil were estimated. The soil moisture contents were converted to pF values. The narrow band values of the laboratory measurements on reflectance were added to compose broad Thematic Mapper bands. Multiple linear regression on the experimental results produced equations with grass moisture, soil moisture and pF as dependent variables. These variables could be estimated from reflectance values in the TM bands 3, 4 and 5 with respective standard errors of 13.8-16.0%, 8.5% and 0.1%. The pattern of TM satellite data of grassland at one specific acquisition date revealed that it is very likely that remote sensing data could be used to detect drought phenomena, although field observations would still be required. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The effect of organic pollution on local distribution of Nereis diversicolor and Corophium volutator

Distribution patterns of Nereis diversicolor and Corophium volutator in the SE-Dollard, a 92 km 2 tidal flat area in the NE Netherlands are described. Body condition as well as the maximal size of juvenile worms correlated positively with elevation of the mudflat, probably caused by a better food supply on the higher mudflats. No spatial variation in size was found for C. volutator. N. diversicolor disappeared from ∼1 km 2 of the intertidal mudflats in the autumn of 1985 as a consequence of the seasonal waste discharges, whereas C. volutator disappeared from an area of ∼4 km 2. Though the discharges decreased by almost 50% from 1982 to 1985, N. diversicolor disappeared from about the same area in the 2 years. N. diversicolor is the main prey species of the avocet Recurvirostra avosetta, large flocks of which stay in the Dollard during autumn migration. Though the avocets lost ∼30% of their potential feeding area in the Dollard in the 1970s because of the high waste discharges, against only 5 to 10% in 1982 and thereafter (at significant lower discharges), the numbers of the avocets did not respond to the decrease of the discharges.